1. Field of the Invention
The present invention relates to a switching power supply that generates reduced conduction noise in switching process of a switching element.
2. Description of the Related Art
A switching power supply is basically composed, as shown in FIG. 5, of a main body SW of the switching power supply and a control circuit CONT. The main body SW conducts switching an input voltage Vin by a switching element Q and delivers a specified output voltage Vout through an isolation transformer T. The control circuit CONT, which is a control IC, ON/OFF-drives the switching element Q at a predetermined frequency. The switching element, which can be a MOS-FET or an IGBT, is connected to a primary winding P1 of the isolation transformer T and conducts switching the input voltage Vin. With the switching of the switching element Q, an alternating voltage is generated on a secondary winding S of the isolation transformer T. The AC voltage is rectified by a diode D and smoothed by an output capacitor Cout to deliver an output voltage Vout. The symbol Cin in the figure designates an input capacitor.
The control circuit CONT works with a driving power source of the winding voltage developed across an auxiliary winding P2 of the isolation transformer T. The control circuit CONT controls the switching of the switching element Q corresponding to the feedback information from an output voltage detecting circuit VS that detects the output voltage Vout. The switching control is performed, for example, by PWM control that changes the ON width of the switching element Q that is driven at a constant frequency, that is, at a constant period, corresponding to the feedback information. The control circuit CONT is also provided with a protection circuit (not shown in the figure) that detects an overcurrent flowing through the switching element Q via a shunt resistor RS series-connected to the switching element Q and protects the switching element Q from overcurrent.
The switching power supply having the above-described construction inevitably generates conduction noise of higher harmonics caused by the switching operation of the switching element Q. In order to reduce the conduction noise, Patent Document 1 (identified below), for example, discloses modulating the switching frequency fs of ON/OFF driving the switching element Q with a predetermined spread width ±Δf.
The spread control (or jitter control) of the switching frequency fs is conducted as shown in FIG. 6 in the control circuit CONT by a comparator 12 that compares a carrier wave of a triangular wave at a specified frequency fs generated by a carrier wave oscillator 11 with the feedback information FB from the output voltage detecting circuit VS and generates a signal for regulating the ON width of the switching element Q. The comparator 12 delivers an output signal for ON/OFF driving the switching element Q through a gate driving circuit 13, thereby performing ON width control of the switching element Q under the condition of the specified frequency fs.
The spread control (or jitter control) of the switching frequency fs is conducted using a sweep circuit 14 that generates a frequency control signal of a triangular waveform, for example, for a frequency spread width of ±Δf and gives the generated frequency control signal to the carrier wave oscillator 11 to modulate the frequency fs of the carrier wave within the range of the frequency spread width ±Δf. As a consequence, the frequency of the carrier wave is given to the comparator 12 after modulation to fs ±Δf. Thus, the switching frequency fs of the switching element Q is modulated with a frequency spread width of ±Δf.
Such spread control (or jitter control) of the switching frequency fs can reduce the conduction noise caused by switching of the switching element Q by about 10 dB on average in a frequency band of from 150 kHz to 1 MHz, for example.